The present invention relates to signal processing either in an apparatus for performing non-destructive testing of a material object or in an ultrasound apparatus used for medical diagnosis and more particularly to an ultrasound signal processor suitable for digitization.
A conventional ultrasound receiver is comprised of analog delay circuits and adders and it radiates ultrasound to an object to be tested, then receives an echo from the object to be tested by means of a receiving element array and adjusts the delay time between receiving signals to change the direction of a receiving beam. The incident direction of the ultrasound is made to be coincident with the direction of formation of the receiving beam, and outputs of respective array receiving elements are made to be in phase and added together to thereby obtain a total output which is large. Receiving signals from an object other than the target, which are received by the respective array receiving elements, are out of phase from each other and consequently cancelled out to thereby produce a total output which is suppressed. Since signals received by a plurality of receiving elements are made to be in phase and added together in this manner to improve resolution, accuracy of the delay time of analog delay circuits must be increased, giving rise to problems of complicated apparatus configuration and high cost. Accordingly, an apparatus has been proposed which simplifies the apparatus configuration and which is mainly constructed of analog circuits not required to have high delay time accuracy. This is based on ultrasound beamforming or so-called beat-down in which the center frequency of a receiving signal is shifted and delayed and thereafter subjected to an adding processing. More specifically, in the beamformer of ultrasound signals, a signal from an ultrasound receiving element is mixed with a reference signal, precisely controlled with respect to time, so as to be converted into a low frequency signal, wherein the low frequency signal component is delayed by means of a delay circuit. Signals from the respective elements which are thus produced are finally added together. A configuration using this method is shown in FIG. 2.
In FIG. 2, reference numeral 23 designates a transducer of ultrasound, 14 denotes an analog mixer, 6 indicates an analog delay circuit whose delay time is settable, 2 represents an analog adder, 18 designates an analog reference signal generator, and 8 denotes a control circuit for analog delay. Where t represents time, a transmitting signal s(t) having a center frequency .omega..sub.s can be approximated by EQU S(t)=A.sub.0 (t){exp(j.omega..sub.s t)+exp(-j.omega..sub.s t)}(1)
wherein A.sub.0 (t) indicates an envelope form of the transmitting signal and j is imaginary unit. A receiving signal f.sub.n (t) of a targeted echo signal generated from this transmitting signal and received by an n-th array receiving element is given by ##EQU1## where .tau..sub.n is propagation time of the ultrasound. Here A.sub.n =k.sub.n A.sub.0 and k.sub.n is a coefficient determined by a propagation distance of the echo. Multiplication of this signal by a reference signal h.sub.n (t) generated from the analog reference signal generator 18 is carried out by the analog mixer 14. For simplification, it is now assumed that h.sub.n (t) is a signal having the same frequency as a carrier of the receiving signal, and h.sub.n (t) is given by EQU h.sub.n (t)=exp{-j(.omega..sub.s t-.phi..sub.n)} (3)
when a phase term .phi..sub.n is taken into consideration. A multiplication result g.sub.n (t) is ##EQU2## When only a carrier component which is direct current is considered, the multiplication result is expressed by G.sub.n (t) which is EQU G.sub.n (t)=A.sub.n (t-.tau.n) (5).
This waveform is delayed by a time of .tau..sub.0 -.tau..sub.n by means of the analog delay circuit 6 to provide a signal V.sub.n (t) which is ##EQU3## where .tau..sub.0 is a constant determined by the analog delay circuit 6. As will be seen from the above, signal V.sub.n (t) results from time shift of A.sub.n (t), and A.sub.n (t) is constant times as large as the envelope A.sub.0 of the transmitting signal. Therefore the signal V.sub.n (t) has a common waveform whose amplitude scale depends on n. Consequently, in a final result which is obtained by adding thus processed signals by means of the analog adder 2 and which is expressed by ##EQU4## respective signals are in phase with each other and the sum Y(t) grows greatly. On the other hand, in an echo coming from a direction other than the targeted direction, respective signals have phases which are different from .phi..sub.n in equation (3) and a phase term remains in equation (5). This causes interference due to the phase difference occurring during addition pursuant to equation (7) and thus the sum Y(t) damps. Based on the operational principle described above, receiving signals from the targeted direction can be selected. For example, Japanese Patent Publication No. 51068/1985 and U.S. Pat. No. 4,140,022 are relevant to this type of apparatus. The configuration shown in FIG. 2 is typically realized with analog circuits but in order to improve accuracy of beamforming and further enhance the quality of the apparatus, it may preferably be realized with digitized operation units.
Conceivably, the conventional apparatus constructed of analog circuits may be simply digitized with, for example, an apparatus constructed as shown in FIG. 3. In FIG. 3, an analog to digital converter 5 is used and the analog mixer 14, analog delay circuit 6, analog adder 2, analog reference signal generator 18 and control circuit 8 for analog delay shown in FIG. 2 are modified for digitization to provide a digital mixer 15, a digital delay circuit 7, a digital adder 3, a digital reference signal generator 19 and a control circuit 9 for digital delay, respectively, which are used in FIG. 3. In this configuration, the analog to digital (A/D) converter is required to have many bits, for example, 10 bits or more to ensure amplitude accuracy in ordinary applications of ultrasound. Under the circumstances, a configuration is conceivable wherein a so-called over-sampling technique using known fast sampling and cumulation of signals in combination is applied to increase the effective number of bits. For example, in an example of configuration of FIG. 4, a digital adder 4 for cumulation adapted to calculate the sum of a plurality of signals is arranged between the analog to digital converter 5 and digital mixer 15 shown in FIG. 3. The effect of this over-sampling technique depends on the number of cumulating operations but the ultrasound signal has a waveform as shown in FIG. 5, with the result that the time length for permitting cumulation within an amplitude change of .DELTA. or less is limited to T.sub.0 or less and a remarkable improvement in accuracy cannot be expected. In FIG. 5, A(t) represents an envelope having the same form as that of the envelope of the transmitting signal s(t) and this waveform is a typical example in the conventional ultrasonic apparatus.
In the prior art described above, when materializing a highly accurate digital beamforming processing, a problem arose in simplifying the analog to digital converter. Further, it was difficult to improve both the amplitude accuracy and the sampling frequency of the analog to digital converter in compliance with a receiving signal having a high center frequency. Further, there arose a problem that a sampling frequency of the analog to digital converter not sufficiently higher than easily twice the upper limit frequency of a receiving signal component, could not be dealt with.